1. Field of the Invention
The invention relates to a method for turning a three-phase alternating current motor (induction motor or synchronous motor) that drives a work machine (e.g. geared turbocompressor) on again, after this motor has been separated from a supply voltage and if a residual field voltage induced by a rotor residual field is present, as well as to an electrical circuit for implementing the method.
2. The Prior Art
Many drives in different applications require the driving three-phase AC motor to be turned on again as quickly as possible and with a low loss in speed of rotation, after an interruption in the supply network has occurred and the motor has been separated from the supply voltage, but at the same time, damage to the motor, to the machine driven by the motor, and all other design parts of the shaft train (clutches, transmissions, etc.) between the machine and the motor must be prevented. In this connection, it must be taken into consideration that the motor keeps running after the motor has been separated from the supply voltage, and a residual field voltage of the speed of rotation frequency is induced by the rotor residual field that is present. Because of the counter-moment of the driven machine, and because of friction, the motor runs down. In the case of three-phase AC motors, after a network failure has occurred, return of the supply voltage with phase oppositions to the residual field voltage represents an extreme stress with regard to the surge currents and surge moments that occur. In particular, in the case of a high mass inertia moment of the work machine, these surge moments introduced on the motor side can bring about extreme rotational vibration stresses in the entire drive train, so that all the components of the drive train, such as motor, clutch, and work machine must be designed to be correspondingly robust and thus super-sized for normal operation. A calculation method for determining the rotational stress that occurs during a network switch is known from the essay “Archiv für Elektrotechnik [Archive for Electrical Engineering] 71, 1988, pages 399 to 411.”
In order to avoid super-sizing the components of the drive train or the use of an additional slip clutch, it is known from practice to separate the motor, in the case of disruption in the supply network, by opening a switch, and subsequently to connect it with a reserve network, or with the supply network again, after the disruption has been corrected, in a controlled manner, by closing the switch. As presented in the essay “ELEKTRIE, Berlin 46, 1992, pages 456-460,” methods are known for short-time and long-time switching for renewed application of voltage to a three-phase AC motor, if a residual voltage induced by a rotor residual field is present. In order to prevent an excessive torque stress in the drive train in the case of short-time switching, typically within a few milliseconds, the differences between the amplitudes, the frequencies, and the phase shifts of the voltages of a supply network and a reserve network are constantly monitored during operation. Only if all of the differences stay within narrow limits can a switch to the reserve network take place, within a few milliseconds, in case of a disruption of the supply network. If the conditions for short-time switching are not met, long-time switching takes place, waiting until the motor has been braked so far that an excessive torque stress can be precluded when voltage is applied, independent of the phase difference of the supply voltage and the residual field voltage. It is true that the method of long-time switching guarantees moderate surge moments, but it has the disadvantage that the speed of rotation drops so greatly that as a result, the working process is practically interrupted. Finally, in the case of long-time switching, there is also the risk that turning the motor on again takes place at a torsion-critical or bending-critical speed of rotation of the drive train, and great alternating moments occur as a result of the switching process, which endanger the components in the shaft train, or the automatic safety shut-off is actually activated.